The use of remote control to effect a desired operation of various systems and devices is relatively widespread. For example, inaccessible or inconveniently located systems are particularly suited for remote control. Similarly, when a number of widely dispersed systems are involved, remote control can be used to provide the desired operation of each system from a single control point. In either case, control is easily achieved with a minimum of operator time, thus reducing system operating costs.
One way in which reliable remote control has been achieved is with the aid of communication systems. Such communication systems produce a suitably encoded signal, usually by modulating one or more tones of specified frequency and duration onto an appropriate radio frequency (RF) carrier wave, and transmit the signal to the system to be controlled. There, the encoded signal is received and demodulated, with the resultant tones being applied to a decoding apparatus. The decoding apparatus produces an output signal that effects a change in the operation of the remote system only upon receipt of tones having the specified frequency and duration. Otherwise, the operational status of the system remains unchanged.
One particular type of communication system that has found widespread commercial acceptance in the logging industry employs an encoded signal commonly identified as a "two-tone sequential" signal. The encoded signal is used to effect the remote actuation of a whistle or other signaling device to indicate to personnel information concerning the operation of logging equipment. While two such "two-tone sequential" systems are described in detail in U.S. Pat. Nos. Re. 27,044"Two-Tone Remote Control System," Rothenbuhler et al., issued Feb. 2, 1971, to the assignee of the present invention, and 4,197,525, "Tone Decoder," Biery, Jr. et al., issued Apr. 8, 1980, to the assignee of the present invention, their operation is briefly as follows.
A first tone of limited duration and of a specified frequency is modulated upon an RF carrier by a transmitter. Immediately following the termination of the first tone, a second tone having a specified frequency that is different from the frequency of the first tone is modulated upon the RF carrier. The duration of the second tone is equal to the desired actuation time of the remote signaling device and the frequency of both tones is typically in the audible frequency range.
At the location of the whistle, signals having the frequency of the RF carrier are received and demodulated. The demodulated signals are then supplied to a decoder. The function of the decoder is to first identify the occurrence of a demodulated signal having a frequency corresponding to the nominal frequency of the first tone. In that event, the decoder continues to monitor the demodulated signal for the occurrence of a signal having a frequency corresponding to the nominal frequency of the second tone. The decoder provides an output signal to the whistle when the frequency of the demodulated signal corresponds to that of the second tone, but only if such a demodulated signal is detected within a predetermined period of time following detection of the first tone. Such a "two-tone sequential" system provides reliable, remote actuation of the whistle, even in the presence of encoded signals used in other communication systems and having the same RF carrier modulated with a single tone at the nominal frequency of either the first or the second tone.
As will be appreciated from the preceding discussion, it is desirable to provide a tone decoder that is able to detect tones with a high level of precision. More particularly, while the use of "two-tone sequential" encoding is designed to minimize the unintended actuation of a remote system or device, the effectiveness of this technique is proportional to the ability of the decoder to precisely identify the two tones in sequence. Further, it is typically desirable that the decoder respond quickly to encoded information to avoid undue delay in the control of the remote system. Thus, it would be desirable to provide a decoder that is able to both quickly and accurately identify one or more tones encoded upon a carrier wave.